The present invention relates to a magnetic field control system. More particularly, the present invention relates to a control system for controlling the magnetic fields created by the electromagnets in the nozzle of a charged-particle beam delivery system for the purpose of steering the charged-particle beam to follow a desired raster scan pattern across a selected target site.
A beam delivery system typically includes a nozzle through which a charged-particle beam, such as a proton beam, passes on its way to a target area. The nozzle usually includes collimator devices for confining the beam to a desired area or shape on the target. It is known in the art to include an electromagnet(s) in the nozzle that, as controlled by a drive current, generates a magnetic field that interacts with the charged-particle beam so as to bend or deflect the beam in a desired direction(s). By applying a current of a known magnitude to the electromagnet(s) in the nozzle, a magnetic field of a predictable intensity is generated that bends or deflects the beam a predictable amount. The stronger the magnetic field, the more the charged-particle beam can be bent. Hence, by selectively applying a current of a controlled magnitude to the electromagnet(s), the beam can be steered to a desired point on the target area.
The strength of the magnetic field required to bend a charged-particle beam dictates that an electromagnet having a large number of ampere-turns be employed. A low number of turns provides low resistance and inductance, but dictates that a high current be used. Unfortunately, such high currents are difficult to generate and control, and further create heat dissipation problems. A high number of turns advantageously allows the current to be maintained at a manageable level, but unfortunately brings with it a high inductance and resistance. Disadvantageously, a high inductance limits the rate at which the current, and hence the magnetic field, can change. Thus, prior art uses of electromagnets to steer a charged-particle beam have typically been limited to static uses, that is, uses where the steering of the beam is not changing, or is changing at a very slow rate. However, what is needed for a beam delivery system where the target is larger than the beam diameter is a system for dynamically steering a charged-particle beam so that the beam can be scanned across the target in a desired manner.
It is known in the art to use two magnets in the nozzle of a beam delivery system, oriented so as to create orthogonal magnetic fields, and to apply a fixed magnitude voltage to the coil of one magnet in one direction for a prescribed period of time, and then to apply this fixed magnitude voltage to the same coil in the opposite direction for a prescribed period of time, with the rate of change of the current being limited by the inductance of the coil. In this manner, a sawtooth current waveform is passed through the magnet coil, resulting in a magnetic field that linearly changes from one peak value to an opposite peak value. The changing magnetic field causes the beam to bend or move across the target area from one edge to the other in a straight line. When a similar fixed magnitude voltage is applied to the other magnet coil, another magnetic field, substantially independent of the first magnetic field, causes the beam to bend or move across the target area in a direction that is orthogonal to the movement caused by the first magnetic field. The combined effect of both magnetic fields is to move the beam across the target area in a diagonal sweep pattern. Typically, one magnet sweeps the beam in one direction, e.g. a horizontal direction, at a much faster rate than the other magnet sweeps the beam in an orthogonal direction, e.g., a vertical direction. Hence, one magnet is referred to as the "fast magnet" and the other is referred to as the "slow magnet".
Unfortunately, such a diagonal sweep pattern, even when there is a large difference between the sweep rates of the fast and slow magnets, does not always efficiently or uniformly cover the target area, there being significant portions of the target area that are not covered by a diagonal sweep pattern. Further, because of the large current magnitudes involved in such an arrangement, there is no easy way to control the current, especially at high sweep rates, other than to change its direction through the coil by abruptly reversing the voltage applied to the coil. This type of abrupt switching, or "bang-bang" type of control, makes it difficult to accurately control that portion of the target area that is swept with the beam. Either the beam sweep is on, in which case the diagonal sweep pattern goes from one edge of the target area all the way over to the other, or the beam sweep is off, in which case the beam does not move within the target area. What is needed, therefore, is a means for selectively controlling the sweep pattern within the target area so that all or a selected portion of the target area can be efficiently and uniformly swept with a beam that is accurately controlled.